I do not deny that there are many and serious difficulties in understanding
how many of the inhabitants of the more remote islands, whether still
retaining the same specific form or subsequently modified, have reached
their present homes. But the probability of other islands having once
existed as halting-places, of which not a wreck now remains, must not be
overlooked. I will specify one difficult case. Almost all oceanic
islands, even the most isolated and smallest, are inhabited by land-shells,
generally by endemic species, but sometimes by species found elsewhere
striking instances of which have been given by Dr. A.A. Gould in relation
to the Pacific. Now it is notorious that land-shells are easily killed by
sea-water; their eggs, at least such as I have tried, sink in it and are
killed. Yet there must be some unknown, but occasionally efficient means
for their transportal. Would the just-hatched young sometimes adhere to
the feet of birds roosting on the ground and thus get transported? It
occurred to me that land-shells, when hybernating and having a membranous
diaphragm over the mouth of the shell, might be floated in chinks of
drifted timber across moderately wide arms of the sea. And I find that
several species in this state withstand uninjured an immersion in sea-water
during seven days. One shell, the Helix pomatia, after having been thus
treated, and again hybernating, was put into sea-water for twenty days and
perfectly recovered.
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